There are lots of amazing things about eggs, but this morning I’m most impressed by their ability to thicken things. All by itself a raw egg is a thickener. Add it to some sweetened half-and-half, put the mix in the blender, and in a few seconds they egg’s emulsifiers will render a nice thick egg nog. And that’s without enlisting the help of those amazing egg proteins.
Egg proteins make up only about ten percent of the total weight of an egg. Heated and coagulated, however, they turn that egg into an extremely rigid gel (say when it’s hard boiled). To arrive at a silky and tender custard, more liquid must be added to the matrix. A basic firm custard mix (say for a flan) consists of about one cup of milk to one egg — a six-fold increase in liquid. Yet the finished product will still be firm enough to stand up by itself when you set it on a plate. Too cool.
Here I should point out that there is more than one type of egg protein. Eggs in fact contains dozens of them, from firm and fast-setting white proteins (ovomucin, ovotransferrin) to weaker, slower-setting yolk proteins. Each has their place, depending on the type of custard you’re setting out to make.